Other Modifications

There are various other modifications which can be made to the camera, depending on whether you feel like it. Here are some that I have tried.

Removing tails

I am indebted to Robin Leadbeater for this modification.

The output from the 1004x appears to be passed through a high-pass filter. I'm not absolutely sure where this happens (the suspicion is the SAG compensation on the video processor) but the effect is to introduce dark 'tails' to the right of saturated stars. The reverse also happens but is less visible - bright tails to the right of dark objects.

A solution to this problem consists of, effectively, low-pass filtering the video signal to remove the excessive high frequencies. To do this, Robin and I have soldered a 3nF capacitor across (i.e. in parallel with) the video cable at the camera end. Visually this appears to 'soften' the image slightly, but I think this is because it was artificially sharp beforehand. At any rate, it certainly helps with the 'tail' problem and is an easy modification to try.

Increasing gain

The maximum gain that the camera will apply to the video signal can be increased fairly easily. Pin 14 if the CXA1310AQ controls this maximum gain. I connected it, via a 2K2 resistor, to ground. The result is that when the camera is imaging a dark subject, it will increase the gain much further. Noise levels increase too, of course, but this can be got around by cooling or by averaging multiple frames.

I haven't yet determined how useful the increased gain is - whether it actually allows the camera to detect fainter objects or whether it simply makes those faint things brighter - but it looks as though it may be useful

Manual gain control

I have noticed that the long exposure images from the 1004x often show a darkened band around 1/3 the way down the image. I believed this was caused by the automatic gain control (AGC) on the camera settling down after receiving a sudden bright image after a period of dark. I therefore decided to make the gain manually controlled, by lifting pin 13 of the CXA1310AQ, and adjusting the voltage at pin 19 (AGC +IN) using a variable resistor and some fixed resistors. I achieved manual control, but the dark band was still present so I suspect that it may be the AGC on the capture card.

The voltage range that affects the gain appears to be about 1.9 to 2.1 volts, quite a narrow range, which was hard to control accurately with the variable resistor. Since this method failed to eliminate the dark band, I think I may back it out and simply adjust the max gain as described above - allowing the camera to set the gain automatically.